it's all in the details. ask your instructor and he will show you all the fine details to make the pass work. otherwise, you're probably making so many mistakes, that you "feel" that it's not working.
here's a basic double under pass that i use alot.
1. make sure you sit back on your heels and posture. put both hands on his belt or pants. If you're leaning even slightly forward, he will pull you in or sweep you.
2. without letting go of your grips, create space and break his guard by placing your elbows against the inside of his thighs, at the same time, push your spine back (think of the shape of a hooded cobra). Do not lean forward or he will pull you in or sweep you!
3. quickly, get BOTH underhooks around the legs, and establish a palm to palm grip, then suck him into you taking away the space. Make sure that the leg you are going to pass is high on your shoulder. Do not continue to lift him upwards with your grip! you have to keep the pressure down towards the ground.
4. sprawl both legs out putting pressure on him. Do not let go of your grip!
5. with thumb in, grab his lapel and use the other hand to "feed" the collar until it is very high and tight. Do not pull the free arm out or else you will get triangled! maintain both grips.
6. while maintaining the high lapel grip, jam your forearm into his neck. while still maintaining the sprawl, begin slowly "edging" your way around the leg that should still be high on your shoulder. Maintain your shoulder pressure against that leg!
7. now prop your back leg up against the small of his back, and with that foot, begin walking it up from your heel then to your ball of your foot, continue this "creeping" from heel to ball of foot as if you are jacking up a car. Make sure to maintain the shoulder pressure on his leg and forearm pressure on his neck on the side you are passing.
8. if you have done this correctly, he should be stacked and very uncomfortable. He will beg you at this point to pass! Without letting your shoulder pressure off, continue to pass until you have proper side control.
9. Do not let go of the high collar grip and the pressure of your forearm against his neck, since you can transition right from side control into full mount and right into a collar choke from there.
There are many other options along the way, of course. Have fun with it and keep asking your instructor questions.